1. Field of the Invention
This invention relates generally to methods and apparatus for power generation while reducing emission of industrial pollution during such power generation. More particularly, the invention relates to a process and apparatus for: burning carbonaceous materials and especially high sulfur-containing coal and low grade carbonaceous material under essentially stoichiometric conditions in a circulating fluidized bed combustor; reducing SOx and NOx emissions from industrial and utility boilers; and repowering cyclone-fired boilers, pulverized coal-fired boilers and oil and gas-fired boilers. The invention provides means to comply with air pollution standards without excessive capital expenditures.
2. Reported Developments
Fossil fuel-fired boilers utilized in the industry today are complex heat exchange apparatuses, the basic function of which is to convert water into steam for electricity generation and process applications. Coal, ranging from lignite having low BTU values to high-rank coals, such as anthracite, is typically used in these boilers, it being abundant and relatively inexpensive. The physicochemical aspects of coal combustion are complex and depend on parameters such as the coal's elemental composition and the apparatus in which the combustion occurs. For example, low-rank coals having lower BTU values are easier to ignite than high-rank coals; however, low rank coals have a higher moisture content which inhibits combustion and consumes useful heat. The overall heat balance for coal combustion reactions also involves such factors as particle size, surface area, pore structure, volatile matter content, additives and impurities of the coal.
The combustion process in generating power for electricity and other uses also generates undesired products carried in the effluent gases, such as NOx and SOx. Prior art combustion systems are directed to reducing such emissions into the atmosphere while increasing the usable heat values extracted from the coal. The systems in use for most commercial applications of coal combustion are fixed-bed, entrained flow and fluidized bed combustors.
Fixed-bed combustion is characterized as being either up-draught or down-draught combustion both utilizing sized coal particles. In an up-draught configuration the primary air source is at or slightly below the level of the fuel. The fuel is ignited at the bottom and the flame travels upward with the air flow. A secondary air inlet is positioned above the level of the bed to facilitate combustion of volatiles emanating from the bed prior to being combusted. Smoke, containing incompletely combusted volatiles, including harmful pollutants, easily escapes from this system. In the down-draught configuration, the air flows downward onto the fuel bed and the flame front moves counter to the direction of the air and the emanating volatiles are kept in the flame by the air steam. This system achieves a more complete combustion and reduction of pollution than the up-draught configuration.
The entrained flow combustor system utilizes finely pulverized coal and a high velocity carrier, such as air or other gases, to suspend the finely divided coal particles. The operating temperatures are as high as 1400.degree. to 1700.degree. C. The release of heat is greater than that produced by the fixed bed or fluidized bed systems, however, the drawbacks are corrosion problems and high nitrogen oxide emissions.
The fluidized bed process uses sized coal particles which are caused to float in an upward stream of gas. The process uses low operating temperatures in the range of approximately 1500.degree. to 1700.degree. F. which reduces the emission of nitrogen oxides. Efficient combustion can be achieved at this temperature and with as little as one to five percent coal feed. This low coal feed also allows the addition of materials which can greatly reduce emission of other pollutants. Limestone (CaCO.sub.3) or dolomite (CaCO.sub.3 -MgCO.sub.3) are, therefore, used in fluidized bed reactors to remove sulfur pollutants by forming calcium or magnesium sulfates from SO.sub.2 released during combustion. Recovery and recycling of the calcium or magnesium can be achieved by treatment of the sulfates with H.sub.2 or CO to produce sulfur dioxide which is not fugitive and can be used for sulfuric acid manufacture and recovery of elemental sulfur. Loss of fines, however, occurs during fluidized bed combustion and must be controlled with cyclones or electrostatic precipitators incorporated into the system.
Fluidized bed systems are usually classified in terms of: operating pressure, namely atmospheric or pressurized, and fluidization mode, namely bubbling or circulating. The circulating fluidized bed system exhibits higher combustion efficiency and sorbent utilization, lower NOx emission due to multiple air staging and greater fuel flexibility as compared to a bubbling type system. Such a circulating fluidized bed boiler is disclosed, for example, in U.S. Pat. No. 5,255,507. Briefly described, the circulating fluidized bed (hereinafter sometimes referred to as CFB) combustor comprises: a combustion chamber into which a combustible material, such as coal, noncombustible material, such as limestone and primary and secondary air are fed. These materials are maintained in a fluidized state by controlling the bed material and flow of air. The combustion chamber is defined by combustion walls having membrane type tubes incorporated therein to contain circulating water. The water is heated in these tubes to produce steam which, after having been subjected to means to increase its temperature, such as a superheater, is directed to a steam turbine. The steam turbine is connected to an electric generator to produce electric power. The hot combustion output is carried from the combustion chamber to a hot cyclone separator in which the solid particles are separated from the flue gasses and returned to the bottom of the combustion chamber for recirculation.
In power generation for various industrial uses a main objective is to reduce the amount of pollution released into the atmosphere or ash discarded into the environment while achieving high temperatures necessary to the operation of fuel-driven turbines and the like power generating systems. The main atmospheric pollutants incident to power generation are oxides of nitrogen (NOx) and oxides of sulfur (SOx). The oxides of nitrogen are mostly nitric oxide (NO) and nitrogen dioxide (NO.sub.2).
NOx emissions from stack gases through reactions occurring in the air produce smog and contribute to the formation of acid rain. State and federal agencies have been and are setting increasingly stricter limits on the amounts of oxides of nitrogen that can be vented to the atmosphere. To comply with state and federal standards various methods were tried and proposed by the prior art. These methods included, for example, injecting water into the combustion zone to lower the flame temperature and retard the formation of NOx which increases with increasing temperatures; and injecting ammonia and a reducing catalyst to achieve a similar reduction in NOx formation.
To suppress sulfur dioxide generated during combustion of carbonaceous fuel, such as coal and leachable sulfur in the ash residue of combustion, coal is mixed with a sulfur absorbent such as calcium oxide, calcium hydroxide or calcium carbonate prior to combustion or gasification. To achieve satisfactory sulfur capture, the combustion temperature, however, has to be maintained at less than about 1700.degree. F.
To wit, the reduction/elimination of pollutants require the use of relatively low temperature combustion, while efficient power generation requires the generation of high temperatures.
Examples of prior art disclosures trying to satisfy both requirements follow.
U.S. Pat. No. 4,103,646, discloses a fluid bed boiler having two zones: in the first zone coal and limestone are fed, fluidized by air at high velocities and combusted to capture sulfur dioxide; the solids exiting from the first zone is lead into the second, slow bubbling bed zone fluidized by low velocity air. Solids remaining in the slow bed are recirculated back into the first zone. The second zone contains heat exchangers.
U.S. Pat. No. 4,616,576 relates to a two-stage combustion method utilizing first and second circulating fluidized bed systems.
Fuel is supplied to the first circulating fluidized bed system and is combusted therein under reducing conditions of 700.degree. to 1000.degree. C. Solid material is separated from the gases discharged from the first circulating fluidized bed system and recirculated into the first fluidizing bed system. The flue gases are fed into the second circulating fluidized bed system, which contains a sulfur-absorbing agent, such as lime, to effect after burning and to reduce NOx formation.
U.S. Pat. No. 5,156,099 discloses a modified circulating fluidized bed boiler, termed internal recycling type fluidized bed boiler, in which the fluidized bed portion of the boiler is divided by a partition into a primary combustion chamber and a thermal energy recovery chamber. Two kinds of air supply chambers are provided below the primary combustion chamber: one for imparting a high fluidizing speed to a fluidizing medium; and the other for imparting a low fluidizing speed to the fluidizing medium, thereby providing a whirling and circulating flow to the fluidizing medium in the combustion chamber. Exhaust gas is lead to a cyclone and fine particulates collected at the cyclone is returned into the primary combustion chamber or in the thermal chamber.
U.S. Pat. No. 4,936,047 discloses a method for reducing the amount of gaseous sulfur compounds released during combustion of sulfur-containing fuel comprising: mixing the fuel with an aqueous solution of calcium-containing sulfur absorbent; exposing the mixture in a reactor to a reducing atmosphere at a temperature range of 1500.degree. F. to 1800.degree. F. for converting at least 20% of the solid carbonaceous material to the gaseous state while forming a solid char material; and passing the solid char material into a combustor and combusting the char at a temperature of at least 2100.degree. F. in the presence of oxygen to promote the reaction of sulfur to form calcium sulfate.
U.S. Pat. No. 5,178,101 pertains to a method for reducing oxides of nitrogen that are generated in a coal-fired fluidized bed boiler comprising the steps off
flowing an exhaust stream from the fluidized bed boiler generated at a temperature of about 600.degree. to 650.degree. F. through a thermal reaction zone at which fuel and air are burned to produce a modified heated stream;
passing the modified heated stream over a first catalyst bed to oxidize N.sub.2 O and NOx to NO.sub.2 ;
cooling the exhaust stream; and
passing the cooled stream over an oxidizing catalyst bed to oxidize remaining combustibles.
While the prior art have made significant advances in reducing environmentally harmful pollutants during combustion of carbonaceous fuel to generate power, there is still a need for further improvements in pollution reduction and power generation. The present invention is directed to such improvements as described more particularly in the objects of the present invention that follows.
It is an object of the present invention to provide a method for enhancing sulfur capture by sulfur absorbents during the combustion of particulate fuel.
It is another object of the present invention to provide a method to reduce oxides of nitrogen generated during the combustion of carbonaceous fuel.
It is still another object of the present invention to generate power at high efficiency, which is accomplished by methods directed to repowering: cyclone-fired boilers, pulverized coal-fired boilers, gas- and oil-fired boilers; and boilers having conventional radiant furnaces.
A still further object of the present invention is to provide a system for attaining these objects.